nanotechnology application for solar cells: using quantum dots to modify absorption properties...
TRANSCRIPT
Nanotechnology Application for Solar Cells: Using Quantum Dots to Modify Absorption Properties
QUANTUM NANOS INC.
Introduction
How Classical Solar Cells Operate Absorption Coefficient (α)
Definition and Relevance of α Physical Techniques for Measuring α
Light Absorption of Quantum Dot Layers Reasons for Interest Into Quantum Dot Light Absorption Definition of a Quantum Dot Formula for Light Absorption of a Quantum Dot Comparison of α versus Energy for Bulk Material and Quantum Dot
Researchers working on Light Absorption of Quantum Dots Dr. Sheila Baily Dr. Ryne Raffaelle
Problem Statement – Determining the most optically absorbent semiconductor material
Problem Solution Explanation of Theory Results
Absorption Coefficient α –Definition and Relevance of α3
Definition of Absorption Coefficient α A measure of the rate in decrease of
electromagnetic radiation (as light) as it passes through a given substance; the fraction of incident radiant energy absorbed per unit mass or thickness of an absorber.
Absorption Coefficient α –Definition and Relevance of α3
Unit of Absorption Coefficient α The units of α are per length (cm-1)
Absorption Coefficient α –Definition and Relevance of α3
Unit of Absorption Coefficient α The units of α are per length (cm-1)
Absorption Coefficient α –Definition and Relevance of α4
Absorption Versus Transmission Transmission (t): a measure of
conduction of radiant energy through a medium, often expressed as a percentage of energy passing through an element or system relative to the amount that entered.
Absorption Coefficient α –Definition and Relevance of α4
Absorption Versus Transmission Transmission (t): a measure of
conduction of radiant energy through a medium, often expressed as a percentage of energy passing through an element or system relative to the amount that entered.
Absorption Coefficient α –Definition and Relevance of α4
Absorption Versus Transmission Transmission (t): a measure of
conduction of radiant energy through a medium, often expressed as a percentage of energy passing through an element or system relative to the amount that entered.
0 0.2 0.4 0.6 0.8
2
4
6
8
1010
0
t( )
10 t
Absorption Coefficient α –Physical Techniques for Measuring
α5,6
Optical Transmission Measurement t – Measured transmission l – Sample thickness R - Reflectance
t1 R( )
2e
l
1 R2e2 l
Light Absorption of Quantum Dots – Why We Are Interested7,8,13
These structures have great potential for optoelectronic applications, one of which may be solar cells
Standard solar cells have a theoretical upper conversion rate of 33%, the theoretical limit on the conversion of sunlight to electricity is 67%
Light Absorption of Quantum Dots – Formula7
_
Vav = Average Dot Volume
pfi = 2d momentum matrix element
a = polarization of light
N(ћω) = density of states
Light Absorption of Quantum Dots – Formula12
Transmission for Quantum dots. For transmission through n planes of dots, each
having the same dot density N and each dot experiencing the same optical field amplitude, the transmission fraction is:
Tn=(1-σN)n ≈ (1-nσN) ; (σN << 1) σ represents a cross section of the layer
Researchers Working on Light Absorption of Quantum Dot Layers
Dr. S. Bailey Using quantum dots in a solar cell to create an
intermediate band IEEE Photovoltaic Specialist Conference (PVSC)
Executive Committee since 1987
Researchers Working on Light Absorption of Quantum Dot Layers11
Dr. Ryne R RIT NanoPower Laboratories Organic and Plastic Solar Cells Combined
with Quantum Dot Layers
Problem Solution – Explanation of theory
z = propagation direction nr = refractive index omega = frequency alpha = absorption coefficient Laws of Conservation
Energy Momentum
)2
exp()](exp[z
tc
niEE r
o
Figures based on Singh textbook
Photon Absorption
Photon Emission
)(|)*(|1 2
2
2
cvif
oro
Npamcn
e
32
2/12/3* )()(2)(
gr
cv
EmN
cvif ppa 22
32|)*(
eVm
p
o
cv24~20
2
Problem Statement – Determining the most optically absorbent semiconductor bulk
Consider InP and GaAs as being the available semiconductors to create a solar cell. This solar cell will be a hybrid, consisting of a traditional solar cell created with either InP or GaAs, and coating layers of quantum dots of either InP or GaAs. If maximizing absorption is the only criteria for designing the solar cell, which material should be used for the bulk? Which should be used for the quantum dot layers? Assume the density of states for quantum dot layers of both materials is equal and occurs at the same point, E = .1eV, and that the polarization-momentum product sum is the same in both cases.
Problem Statement – Determining the most optically absorbent semiconductor bulk
Absorption coefficient of InP and GaAs
Required constants by material14
Material Electron Mass (mo)
Hole Mass (mo)
Calculated reduced mass (mo)
Eg(eV)
Lattice Constant(A)
Refractive index (nr)
Gallium Arsenide, GaAs
0.067 mhh* = 0.45 mr
* =0.058 1.5 5.65 3.65
Indium Phosphide, InP
0.073 mhh* = 0.45 mr
* =0.058 1.34 5.87 3
References
1. Seale, Eric. Solar Cells: Shedding a Little Light on Photovoltaics. 28, Feb. 2002. Solarbotics. <http://www.solarbotics.net/starting/200202_solar_cells/200202_solar_cell_physics.html>. Accessed 03/20/2005.
2. Pierret, Robert F. Semiconductor Device Fundaments. Addison Wesley Longman, 1996. pp 198-205.3. Anonymous. Absorption Coefficient. Undated. LaborLawTalk.
<http://dictionary.laborlawtalk.com/absorption_coefficient>. Accessed 04/01/2005.4. Anonymous. Transmission (T). Undated. Photonics Directory.
<http://www.photonics.com/dictionary/lookup/XQ/ASP/url.lookup/entrynum.5189/letter.t/pu./QX/lookup.htm>. Accessed 04/01/2005.
5. Augustine, G. Jokerst, N.M. Rohatgi, A. Absorption measurements of doped thin film InP for solar cell modeling. IEEE: Indium Phosphide and Related Materials, 1992., Fourth International Conference on. 21-24 April 1992.
6. Gerber, D.S. Maracas, G.N. A simple method for extraction of multiple quantum well absorption coefficient from reflectance and transmittance measurements. Quantum Electronics, IEEE Journal of. Volume: 29 , Issue: 10. Oct. 1993.
7. Kochman, B; Singh, J; et al. Absorption, Carrier Lifetime, and Gain in InAs-GaAs Quantum Dot Infrared Photodetectors. IEEE Journal of Quantum Electronics. Volume 39, Number 3. March 2003.
8. Anonymous. Photovoltaics. Evident Technologies. Undated. <http://www.evidenttech.com/applications/quantum-dot-solar-cells.php>. Accessed 04/14/2005.
9. Singh, J. Modern Physics for Engineers. John Wiley & Sons, Inc. 1999. pp 34, 156.10. Wu, Y. Singh, J. Polar Heterostructure for Multifunction Devices: Theoretical Studies. IEEE
Transaction on Electron Devices. VOL. 52, NO. 2, FEBRUARY 200511. Raffaelle, R. Profile of Ryne P. Raffaelle. RIT Department of Physics. Undated.
<http://www.rit.edu/~physics/facstaff/profiles/raffaeller.shtml>. Accessed 04/10/2005.12. Blood, P. On the Dimensionality of Optical Absorption, Gain, and Recombination in Quantum-Confined
Structures. IEEE Journal of Quantum Electronics. Vol. 36, No. 3, March 2000.13. D. Pan, E. Towne, and S. Kennerly. Strong normal-incident infrared absorption and photo-current
spectra from highly uniform (In,Ga)As/GaAs quantum dot structures. IEEE Electronic Letters. 14th May 1998 Vol. 34 No. 10.